Working Principle of Vortex Flow Meter

Vortex flow meters utilize a bluff body or cylinder mounted in a pipe spool that creates alternating vortices behind the cylinder. The frequency of the alternating vortex is proportional to the fluid velocity. Vortex flow meters have no moving parts to maintain or repair, and the signal is read electronically and simply converted to a flow rate. Vortex meters work well with most clean fluids and have similar application ranges to DP flow meters. Vortex flow meters are also referred to as vortex shedding flow meters or oscillatory flow meters. These types of flow meters are used to measure the vibrations of the downstream vortexes caused by an obstruction in the flowing stream. Each obstruction has a vital liquid flow speed at which vortex shedding takes place. This vortex shedding occurs at the instant when alternating low pressure zones gets created in the downstream. These sporadic pressure zones enable the barrier to move towards the low pressure zone. By means of sensors gauging the vortices the flow rate can be easily detected. Hence,

Major components of a vortex flow meter include

A bluff body strut-installed across the flow meter bore
A sensor to indicate the presence of the vortex and to produce an electrical impulse
A signal amplification and conditioning transmitter which gives an output proportional to the flow rate

Vortex flow meter Main Features

Vortex meters are equally appropriate for flow rate or flow totalization measurements.

Use of vortex meters is usually not preferred for slurries or high viscosity liquids. Also their usage is not suggested for batching or other intermittent flow applications.

Since there is rise in viscosity with the drop in Reynolds number, vortex flow meter range ability degrades as and when the viscosity increases. The maximum viscosity limit, as a function of permissible accuracy and rangeability, is found to be somewhere between 8 and 30 centipoises.

In case of gas and steam services, one can gain rangeability better than 20:1 whereas in low-viscosity liquid applications, rangeability offered by a properly sized vortex meter is over 10:1.

With Reynolds numbers more than 30,000, inaccuracy of majority of vortex flowmeters is 0.5-1% of rate.

Vortex metering error increases with the decreasing Reynolds number.

Vortex flowmeters are available in typical flange sizes ranging from 1/2 in. to 12 in.

Wafer body vortex flowmeters i.e. flange less flowmeters are inexpensive as compared to flanged meters. However, flanged meters are considered ideal for applications where the process fluid is perilous or is at a high temperature.

Nowadays, nearly all vortex meters make use of piezoelectric or capacitance-type sensors to determine the pressure oscillation around the bluff body

Vortex Shedding Frequency

The vortex shedding frequency is the vibrating frequency of the vortex shedding. It is the frequency which is directly proportional to the velocity of the liquid flowing in the pipe, and hence to volumetric flow rate. It is independent of fluid properties such as density, viscosity, conductivity, etc., except that the flow must be turbulent for vortex shedding to occur. The basic relationship between vortex shedding frequency and fluid velocity is given below:

St = f(d/V)

In the above equation, St represents the Strouhal number which is a typical dimensionless calibration factor used to differentiate a variety of bluff bodies.

Other parameters are

F = vortex shedding frequency

d = Width of the bluff body

V = Average fluid velocity.

The Strouhal number is generally defined as the ratio of the interval between vortex shedding (l) and width of the bluff body (d). If the Strouhal number of two different bluff bodies is equal, then they will work in the same manner.

Installation

If the vortex flowmeter will be installed in a vertical orientation

Install upward or downward flow for gas or steam. Install upward flow for liquids. For steam and fluids with small solids content, it is recommended to have the flowmeter installed with the electronics to the side of the pipe.

Size Ranges Available :- 0.5 to 12 in. (13 to 300 mm).

Design Pressure :-2000 PSIG (138 bars)

Design Temperature :- − 330 to 750 ° F ( − 201 to 400 ° C)

Materials of Construction:- Mostly stainless steel, some in plastic.

Inaccuracy :- 0.5 to 1% of rate for liquids, 1 to 1.5% of rate for gases and steam with pulse outputs; for analog outputs, add 0.1% of full scale.

Price :- Plastic and probe units cost between $250 and $1500; stainless steel units in small sizes cost about $2500; insertion types cost about $3000